1. Field of the Invention
The present invention relates to a quantization functional device utilizing a resonance tunnel effect, a quantization functional apparatus using the same, and a method for producing the same. In particular, the present invention relates to a resonance tunnel diode, a memory using the same, and a method for producing the same.
2. Description of the Related Art
Recently, quantization functional devices utilizing a quantum effect have been actively studied. As practically usable quantization functional devices, devices which utilize a resonance tunnel effect of electrons such as resonance tunnel diodes have been proposed. A resonance tunnel diode needs a double-barrier structure, in which a quantum well having a size close to that of the de Broglie wavelength of electrons is interposed between tunneling barriers. In particular, a resonance tunnel diode which utilizes a heterojunction formed in compound semiconductor materials has been proposed.
In general, such a resonance tunnel diode is produced by sequentially forming compound semiconductor thin films on a compound semiconductor substrate using crystal growth, wherein each thin film has a thickness corresponding to several atoms. See, for example, Reona Ezaki and Hiroyuki Sakaki, Superlattice Hetero Structure Device, Kogyo Chosakai, pp. 197-252 and pp. 397-435 (1988).
Compound semiconductor materials are typically deposited by molecular beam epitaxy (MBE). One exemplary method for producing a conventional resonance tunnel diode using compound semiconductor materials will be described with reference to FIGS. 40A through 40D. In this specification, all the numerical figures representing thicknesses, concentrations, etc. are given as approximate values.
First, as shown in FIG. 40A, on a first Si-doped GaAs layer 11, a first AlGaAs layer 12 is grown to a thickness of about 2.3 nm. Next, a GaAs layer 13 having a thickness of about 7 nm (FIG. 40B), a second AlGaAs layer 14 having a thickness of about 2.3 nm (FIG. 40C), and a second Si-doped GaAs layer 15 (FIG. 40D) are sequentially formed on the first AlGaAs layer 12. In this manner, a resonance tunnel diode having a double-barrier structure of the first AlGaAs layer 12/GaAs layer 13/second AlGaAs layer 14 is produced.
On the other hand, as a double-barrier structure formed of silicon materials, a structure including a silicon substrate, and a silicon oxide layer and a polycrystalline silicon layer formed on the silicon substrate has been proposed. See, for example, Kazuo Saki et al., Extended abstracts (The 52nd Autumn Meeting, 1991), The Japan Society of Applied Physics, No. 2, page 653, 10a-B-3, entitled "Resonance Tunnel Effect in the SiO.sub.2 /Si/SiO.sub.2 Double-Barrier Structure".
With reference to FIGS. 41A through 41E, a method for producing a conventional resonance tunnel diode formed of silicon materials will be described.
First, an n-type silicon substrate 21 (FIG. 41A) is treated with dry oxidation performed at a temperature of about 1000.degree. C. to form a first silicon oxide layer 22 having a thickness of about 3 nm to about 4 nm (FIG. 41B). Next, a polycrystalline silicon layer 23 having a thickness in a range between about 8 nm to about 12 nm is formed on the first silicon oxide layer 22 (FIG. 41C). The resultant body is treated with dry oxidation performed at a temperature of about 1000.degree. C. to form a second silicon oxide layer 24 having a thickness of about 3 nm to about 4 nm (FIG. 41D). Thus, a double-barrier structure is formed. Then, aluminum is deposited in a vacuum on the second silicon oxide layer 24 to form an aluminum electrode 25 (FIG. 41E). In this manner, a resonance tunnel diode having a double-barrier structure of the first silicon oxide layer 22/polycrystalline silicon layer 23/second silicon oxide layer 24 is produced.
The above-described conventional resonance tunnel diodes have the following problems.
(1) In a resonance tunnel diode formed of compound semiconductor materials, the height of the tunneling barrier is as low as about 1.5 eV or less, and thus electrons cannot be confined in a quantum well sufficiently. Due to the insufficient confinement of the electrons, some electrons transmit through the double-barrier structure even when the electron energy is not in a resonant state in the quantum well. Accordingly, the P/V (peak current/valley current) ratio in the I-V characteristic of the resonance tunnel diode cannot be made sufficiently large. The valley current refers to the minimum current in the I-V characteristic.
(2) In a resonance tunnel diode formed of silicon materials, the quantum well tends to have a relatively poor crystallinity. Therefore, the quantum level in the quantum well is not sufficiently sharp (i.e., the energy level broadens to a certain extent), and thus a satisfactory negative resistance characteristic cannot be obtained.